According to Valve, that strong spike was a 2,000% increase in simultaneous players in the game. Additionally, the game held the top spot on Steam's bestseller list this past week. The extended promotion means that UT3 Black—which consists of the base game and the Titan Pack expansion—will continue to cost just $11.99, and will be free to play again this weekend.

This announcement just goes to show something we already knew: new
content and reduced prices can go a long way towards breathing new life
into a game. After over a year on the market, UT3 was still
able to become a bestseller thanks to a strong promotion on Steam.
Unfortunately, there is still no word as to when the PS3 version of the
game will be getting the updates.